In an LTE-Advanced (LTE-A) system, resources of a plurality of LTE carriers have to be linked for use, which is referred to as multi-carrier aggregation, so as to support a wider system bandwidth, e.g., 100 MHz, than that of the LTE system, where each of the LTE carriers is referred to a component carrier. Multi-carrier aggregation particularly relates to three approaches:
(1) Contiguous LTE carriers across a plurality of frequency bands are aggregated for a wider transmission bandwidth of the LTE-A;
(2) Non-contiguous LTE carriers across a plurality of frequency bands are aggregated for a wider transmission bandwidth of the LTE-A. FIG. 1 illustrates an example of non-contiguous multi-carrier aggregation, where LTE carriers non-contiguous across four frequency bands are aggregated for a transmission bandwidth of the system.
(3) Some of component carriers for multi-carrier aggregation are contiguous and some others are non-contiguous across frequency bands.
At present, the research of the standardization organization demonstrates a preferred solution in which a design over each component carrier is kept as consistent as possible with LTE Release 8 (R8) to thereby ensure normal operation of an R8 terminal over each component carrier.
In the TDD mode of LTE Release 8, asymmetric amounts of traffic required over the uplink and the downlink are accommodated by different uplink and downlink configurations, and Table 1 depicts various configurations of allocating uplink and downlink sub-frames. In a TDD system, a base station has to broadcast information on current allocation of uplink and downlink sub-frames in a current cell to the cell, and a TDD terminal can perform normal transmission and reception of data only upon reception of the information.
TABLE 1Different Uplink and Downlink Configuration Schemesof LTE-TDD SystemLength ofUplink-Time ofDownlinkTransitionCon-fromfigurationDownlinkSub-frame NoSchemeto Uplink012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
In an Orthogonal Frequency Division Multiplexing (OFDM) system, a Cyclic Prefix (CP) typically with a larger length than the maximum delay spread of a radio channel is used to obviate a multi-path effect over the radio channel and thus prevent multi-path incurred inter-symbol interference. Different lengths of a cyclic prefix are adopted to support different channel scenarios, that is, a long CP is used for a scenario with a large multi-path delay (e.g., a scenario with large coverage) and a short CP is used for a scenario with a small multi-path delay (e.g., a scenario with small coverage).
Two types of CP, i.e., a normal CP and an extended CP, with corresponding specific parameters as depicted in Table 2 are defined for uni-cast transmission in LTE R8, where a sub-frame of the normal CP is configured to include seven OFDM symbols including the first one with a CP length of 160 and the other ones with a CP length of 144, and a sub-frame of the extended CP is configured with OFDM symbols each with a CP length of 512. As can be apparent, the extended CP with a larger length offers better multi-path resistance while the number of OFDM symbols available for data transmission is reduced correspondingly per unit of time (e.g., per sub-frame), which means a larger overhead of the system. Therefore a CP configuration scheme has to be performed as needed to deploy a network in practice. The CP configuration scheme refers to whether the normal or extended CP is adopted for the CP of a sub-frame.
TABLE 2ConfigurationThe Number of OFDMTypeSymbols in Sub-frameNsymbULCP Length NCP, lNormal CP7160 for l = 0 (The First OFDM Symbol in Sub-frame)144 for l = 1, 2, . . . , 6 (The other OFDMsymbols in Sub-frame)Extended CP6512 for l = 0, 1, . . . , 5
CP configuration schemes of uplink and downlink signals differ in LTE R8. The downlink CP configuration scheme of a cell is detected by a UE through searching for the cell, and particularly, a delay relative to a synchronization signal is detected, and if the delay is larger than a preset threshold, then the long extended CP is adopted; otherwise, the short normal CP is adopted. The uplink CP configuration scheme of the cell is broadcasted from the cell to the UE.
In summary, the issue of uplink and downlink configurations over different carriers in the TDD mode has not been considered in the current LTE-A system design so that a relevant solution is absent and normal operation of an R8 terminal over each component carriers can not be ensured. Furthermore, a CP length configuration scheme of each component carrier has to be considered in the LTE-A system with introduction of multi-carrier aggregation, but just a uni-carrier CP length configuration is considered for a CP configuration scheme adopted in the LTE R8 system, thus making it impossible to be easily applied to the LTE-A system.